The present invention relates to a device for driving a guest-host type liquid crystal which is made by adding a pleochroic dye to a nematic-cholesteric phase transition liquid crystal or a chiralnematic phase transition liquid crystal, and more particularly to a driving device for a matrix-type display panel making use of a holding effect of the guest-host type liquid crystal.
Such phase transition liquid crystals as above exhibit a hysteresis phenomenon between the relative brightness thereof and the root mean square value of a voltage applied thereto. This hysteresis phenomenon is referred to as the holding effect, which is utilized as follows. Each liquid crystal cell is first applied with a holding voltage having a voltage value which is intermediate between the rising and falling voltages of a hysteresis loop, and a desired cell is then applied with a write-in voltage to be subjected to a phase transition. When the voltage applied to the desired cell is restored to the original holding voltage, the cell is maintained as it is. In other words, these liquid crystals, as is shown in FIG. 1 of the accompanying drawings, can assume at the holding voltage two stable states, in one of which the cell is put in the focalconic phase and scatters light, and in the other state, the cell is put in the nematic phase and becomes transparent. Therefore, it becomes possible by making use of the above hysteresis phenomenon to display a picture image, to increase a phase transition speed, and to prolong a memory time (holding time).
A conventional device for driving a matrix-type liquid crystal display panel, which makes use of the holding effect of the phase transition liquid crystals, is disclosed in an article entitled "Pulse-Length Modulation Achieves Two-Phase Writing in Matrix-Addressed Liquid Crystal Information Displays", by K. H. Walter and M. K. Taner, IEEE Transactions on Electron Devices, Vol. ED-25, No. 2, February 1978, pp. 172 to 174. In this method, a two-phase operation is employed, and each of the holding (sustaining), the write-in and the erasing voltages is formed through pulse-length modulation. In more detail, as is shown in FIG. 2, a voltage V.sub.H having a root mean square value V.sub.o .sqroot.1/N (where V.sub.o indicates a peak value, and N the number of scanning lines) is used as the holding voltage. Further, in writing operation, a selected point is applied with a voltage V.sub.s having a root means square value V.sub.o .sqroot.2/N to assume the focalconic phase, and a non-selected point is applied with V.sub.NS equal to OV to assume the nematic phase. Furthermore, a voltage V.sub.E having a high root mean square value V.sub.o .sqroot.(1-1/N) is used to erase information. As is evident from the above, the ratio of the voltage V.sub.s applied to the selected point in the writing operation to the holding voltage V.sub.H (V.sub.S /V.sub.H) is equal to .sqroot.2. Since the ratio V.sub.S /V.sub.H is fixed as above, it is impossible to set the contrast of picture image to an optimal value by appropriately selecting the ratio V.sub.S /V.sub.H in accordance with the characteristic of a liquid crystal cell used. Further, when the display panel becomes large-sized and the number of scanning lines is increased, the addressing time has to be reduced. In such a case, according to the above method, it is required to employ a high peak value V.sub.o, and therefore it is impossible to drive the display panel by a low voltage circuit using a complementary MOS large scale integration (CMOSLSI).
Further, in a case where a large number of scanning lines are employed in a matrix-type liquid crystal display device, there appears the flicker on a display plane even when the display device is driven with a one-line-at-a-time scanning system, since the liquid crystal is generally slow in response speed. In order to prevent the above flicker, a rapid scanning mode for displaying a picture image has been employed in which the one-line-at-a-time scanning is repeatedly conducted to reduce the flicker by virtue of accumulative response effect. The rapid scanning mode is generally employed together with an amplitude selective multiplexing method, for the purpose of avoiding crosstalk, as is described in a U.S. Pat. No. 3,976,362. In this driving method, however, the number of scanning lines cannot exceed a limit, since the flicker is generated when the frame frequency of the one-line-at-a-time scanning is decreased to a value because of an increase in number of scanning lines, and since it is not possible to make the peak value of the voltage applied to the liquid crystal larger than a value for the purpose of enhancing the frame frequency.